We have measured baroreflex-cardiac sensitivity in patients with essential hypertension and the hyperdynamic circulation syndrome. We noted decreased baroreflex sensitivity in the patients, with an inverse correlation between baroreflex sensitivity and plasma norepinephrine. We have used laser-Doppler flowmetry to demonstrate the absence of reflexive skin microvascular responses in patients with regional sympathectomies. Spontaneous oscillations in skin microvascular flow were observed in the sympathectomized human limbs. We tested the validity of impedance cardiography for non-invasive measurement of cardiac output and stroke volume by comparing the impedance technique with invasive procedures. We are developing a Doppler-ultrasound method for continuous measurements of vascular flow and distensibility. We have tested a circulatory model which predicts the changes in the brachial artery pressure waveform during vasoconstriction and vasodilation. Young patients with essential hypertension have defective modulation of the brachial arterial dicrotic wave, which the model explains in terms of increased arterial rigidity and decreased vasodilator responsiveness.